A hydrokinetic torque converter transaxle is described in U.S. Pat. No. 4,509,389, which is assigned to the assignee of my invention. That transaxle includes a torque converter having an impeller and a turbine. The housing for the impeller includes a lockup clutch assembly having a clutch plate that engages a friction surface on the impeller housing. The clutch plate carries friction material that establishes a frictional driving connection between the impeller and the turbine when the pressure differential across the clutch plate is sufficient to establish a clutch engaging force.
The clutch plate is connected through a damper assembly to the hub of the turbine, thus establishing a mechanical torque transfer between an engine crankshaft and the turbine shaft which bypasses the hydrokinetic torque flow path through the torque converter.
Converter lockup clutch constructions are well known in the art, an early prior art teaching being described in U.S. Pat. No. 3,252,352, which is assigned to the assignee of this invention. Another example of early prior art torque converter assemblies having a lockup clutch is disclosed in U.S. Pat. No. 3,541,893.
More recent prior art teachings include means for controlling the application of the torque converter clutch by means of an electronic controller that establishes a modification of the clutch engaging force under certain operating conditions-for example, during shifts when it is desired to eliminate undesired torque fluctuations and engine speed changes during transient periods when torque flow interruption is desired. The electronic controller establishes a pressure force on the clutch plate that is adequate to meet the transient torque transmission requirements of the driveline. Examples of this are shown in U.S. Pat. Nos. 4,560,043 and 4,301,900.
Prior art U.S. Pat. No. 4,541,893, which also is assigned to the assignee of this invention, includes a clutch capacity modulator valve that establishes a desired pressure in the clutch pressure control chamber defined by the impeller housing and the clutch plate. The controls for establishing the clutch capacity is intended to eliminate excess torque capacity so that the clutch will be maintained in the engaged condition under driving conditions when clutch engagement is desired, but excess clutch capacity is avoided by controlling the pressure differential across the clutch plate. This contributes to more precise lockup clutch control and eliminates undesired torque fluctuations upon clutch application and release.
It also is known in the prior art to effect a continuous slipping of a lockup clutch or bypass clutch for a hydrokinetic torque converter by continuously modulating the pressure that controls the clutch. An example of a continuously slipping bypass clutch that is actuated by the pressure in the torus circuit of the converter is disclosed in prior art U.S. Pat. No. 4,468.988. Another example is disclosed in U.S. Pat. No. 4,662,488.
In those instances when the clutch is allowed to slip continuously rather than engaging to establish a full lockup condition, a relatively large heat dissipation is necessary because of the heat energy that is developed by reason of the dynamic friction torque. This tends to cause instability of the clutch because the annular friction surfaces of the clutch tend to go out of flat. Further, heat spots develop in the impeller housing. Most of the heat that is generated because of the slipping condition of the friction surfaces is transferred directly into the cover for the impeller. Furthermore, the friction surface on the cover requires precise machining and polishing during manufacture. This makes it necessary to use metal stock thickness greater than otherwise would be needed.
The prior art designs also are deficient with respect to the ability of the converter to transfer heat energy into the fluid within the torus circuit. Also the instability of the clutch due to heat distortion makes controlling of the clutch torque capacity difficult.